1. Field of the Invention
The present invention relates to a novel process for preparing 4-acyloxy-2-azetidinone derivatives which are useful as intermediates for synthesizing penems or carbapenems.
2. Description of the Background Art
3-[1'-(R)-hydroxyethyl]-4-acyloxyazetidinone as well as its derivatives with the hydroxy group or the .beta.-lactam NH group being protected by any one of various protecting groups are used as excellent intermediates for synthesizing penems or carbapenems. Several reports have appeared on the processes for preparing these compounds [e.g. N. Ueyama et al. Japanese Patent Laid-open No. 84057/1987; M. Shiozaki et al. Tetrahedron Lett., 22, 5205 (1981)].
Processes for preparing 3-[1'-(R)-hydroxyethyl]-4-arylthioazetidinone derivatives are also reported (e.g. M. Ishiguro et al. Japanese Patent Laid-open No. 207373/1986; S. Gerard et al. Japanese Patent Laid-open No. 97280/1986 and M. Shibazaki et al. Japanese Patent Laid-open No. 44355/1984). These arylthio derivatives are usually converted into 4-acyloxy or 4-arylsulfone compounds, which are used for the preparation of 3-[1'-(R)-hydroxyethyl]-4-acyloxyazetidinone or its derivatives. Conversion into compounds with an acyloxy group is more desirable because of their higher reactivity. A process using a mercury salt is reported for converting the arylthio derivatives into compounds having an acyloxy group (A. Yoshida et al. Chem. Pharm. Bull, 29, 2899).
The process using a mercury salt is, however, not suitable for industrial application because of the toxicity of mercury salts. There has therefore been a strong need for the development of a process using another less toxic compound.
The present inventors have undertaken extensive studies in order to develop a process for converting 3-[1'-(R)-hydroxyethyl]-4-arylthioazetidinone into the more useful 3-[1'-(R)-hydroxyethyl]-4-acyloxyazetidinone without using a mercury salt. The studies have led to the completion of the present invention.